Eight characters is eight words. The year, month, day and hour of birth can be expressed in eight words, that is, eight characters. China used heavenly stems and earthly branches in ancient chronology. For example, 20061noon on October 25th, 12, 2006 is the year of Xu Bing, commonly known as the year of the dog. 10 is Jihai Month, 25th is Wuyin Day, and noon is Wuyin Day. These eight words are:
Xu Bing Ji Hai has no sound or dance.
Everyone has his own specific time of birth, so he has his own constellation.
In ancient times, when a man and a woman got married, both sides had to match the eight characters. If they get married, then their marriage can be completed. If you don't combine them, it's impossible. Later, people extended this to describe a person who didn't do anything, saying that the horoscope was not intact.